This invention relats to a magnetic recording medium, and more specifically to a magnetic recording medium which exhibits good shelf stability and running performance in high temperature and humidity environments in addition to improved magnetic powder dispersion and surface properties of the magnetic coating film.
In general, a magnetic recording medium such as magnetic tape, etc. is made by coating a base of polyester film or the like with a magnetic coating material prepared by dispersing a magnetic powder, for example, finely divided .gamma.-Fe.sub.2 O.sub.3, in a high-molecular resinous binder. The properties of the magnetic coating film have much to do not only with the running performance and other physical properties of the tape but also with the magnetic powder dispersion and magnetic properties of the medium. The magnetic film properties, in turn, are largely dependent on the composition of, and the additives used in, the resinous binder. In view of these, many different resinous binder ingredients have hitherto been proposed. For instance, vinly chloride-vinyl acetate-vinyl alcohol copolymers have been widely used as a binder resin ingredient for magnetic tapes. This type of copolymers is characterized by relatively low cost, ready solubility in solvents, good compatibility with other resins, and affinity for magnetic particles on account of the hydroxyl group in the molecule which facilitates the dispersion of the particles. Additional advantages of the copolymers are excellent magnetic properties they contribute, including high degrees of orientation and maximum residual magnetic flux density.
The recent trend toward the use of finer magnetic particles has, however, made these resins not always satisfactory. Investigations have therefore been made about increasing the proportion of hydroxyl groups in the copolymers of the foregoing type. As a result, it has already been found that the dispersibility of magnetic particles is properly improved by introducing the OH groups until the OH/CH absorption ratio according to the infrared absorption spectrum reaches 0.7 or more. However, the copolymers with increased hydroxyl group contents have such high glass transition temperatures that they are difficult to process on the surface and hence the resulting magnetic layer after calendering can hardly be improved in surface quality. In an effort to eliminate this drawback, we have previously proposed to employ a copolymer composed of vinyl chloride, a vinyl alkylcarboxylate, another monomer copolymerizable and vinyl chloride, and a saponified vinyl alkylcarboxylate and having an OH/CH absorption ratio according to the infrared absorption spectrum of 0.2 to 0.7. The copolymer, with the proportion of the saponified ingredient kept rather small, is prevented from undergoing a boost in the glass transition point. Partial loss of the dispersibility of the magnetic powder due to the decrease in proportion of the saponified ingredient is made up for with the action of the monomer copolymerizable with vinyl chloride. Thus, improvements are achieved in orientation, maximum residual magnetic flux density, and other characteristics.
Nevertheless, singular use of the copolymer as a binder resin poses problems yet to be solved including inadequate bond strength and questionable storability and running performance of the resulting magnetic tapes in high-temperature high-humidity environments. For this reason the copolymer is blended with a rubbery binder ingredient to provide a binder for magnetic tapes. More recently, ultrafine magnetic particles have come into use, and accordingly the role the rubbery binder is now required to play is not morely to maintain the physical properties of the magnetic tape but, in addition, to improve the dispersibility of the magnetic particles. With this in view, an attempt was made to blend a polyurethane resin having in its molecule a --SO.sub.3 M group wherein M represents an alkali metal atom with the above copolymer. It resulted in satisfactory magnetic particle dispersibility and excellent orientation, maximum residual magnetic flux density, and other magnetic properties. Also, improved surface properties and a striking improvement in the electromagnetic conversion characteristic of the calendered magnetic layer were observed. Still, it has been found that the polyurethane resin having a --SO.sub.3 M group wherein M represents an alkali metal atom cannot give a magnetic tape with high reliability. The resin does not react readily with polyisocyanates and its --SO.sub.3 M group content can have adverse effects upon the properties of the base film (including the elongation, tensilve strength, and breaking strength), thus aggravating the physical characteristics (such as running durability, bond strength, and head abrasion) of the resulting magnetic tape. The present invention is directed to the solution of these problems concomitant with the use of a polyurethane resin having a --SO.sub.3 M group in the molecule.